[FIX] web_editor: firefox editor hidden mass mail

When saving a modified mass mailing, the editor will do a number of
things to improve the mail readability accross mail client.

One of those is replacing font awesome icons by image, but firefox acts
differently than other browser. On a display:none iframe, doing
.css('color') or .height() on an element returns respectively
`undefined` and 0.

This caused an error when getting the color that we could solve by
doing a fallback for firefox like this:

 window.parent.getComputedStyle($font[0]).color

But to get the height() of an element, it seems we always need the
iframe displayed.

With this change, when the iframe is hidden and the browser is firefox,
the code try to display the iframe (with "visibility:hidden;height:1px")
when this part of the code happen.

opw-807180
closes #22639

addons/web_editor/static/src/js/backend.js

@@ -405,7 +405,19 @@ var FieldTextHtml = widget.extend({
                 var layoutInfo = this.editor.rte.editable().data('layoutInfo');
                 $.summernote.pluginEvents.codeview(undefined, undefined, layoutInfo, false);
             }
+            var $ancestors = this.$iframe.filter(':not(:visible)').parentsUntil(':visible').addBack();
+            var ancestorsStyle = [];
+            // temporarily force displaying iframe (needed for firefox)
+            _.each($ancestors, function (el) {
+                var $el = $(el);
+                ancestorsStyle.unshift($el.attr('style') || null);
+                $el.css({display: 'initial', visibility: 'hidden', height: 1});
+            });
             this.editor.buildingBlock.clean_for_save();
+            _.each($ancestors, function (el) {
+                var $el = $(el);
+                $el.attr('style', ancestorsStyle.pop());
+            });
             this.internal_set_value( this.$content.html() );
         }
     },